Photos

Can you tell us why you think you lost photos? From what version of iPhoto were you migrating from - 9.6.1? Have you checked the total number of photos, videos and other items (Photos will also import music files) in the new Photos library (at the bottom of the Photos view)

vs what's in the iPhoto library?

iPhoto 9.6.1 can still open the library with the .migratedphotolibrary extension.

What version of iPhoto did you have?

You only have one iMac, correct? The only thing I can think of is to change the .migratedphotolibrary extension to .ipohotolibrary and do another migration.

If that fails to get all of the files you can open the iPhoto Library package, move the Masters folder to the Desktop and then import the subfolders containing the image files a few at a time into your Photos library. Photos will detect duplicates and give your the choice to import only the new ones:

Were any of the photos in the iPhoto library "referenced" photos? If so those would not be migrated and would not be in the iPhoto library's system.

Can you please explain a bit more? What kind of migration are you asking about? Do you mean the migration from iPhoto to Photos? Do you still have the original library?